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So, I was reading about aneuploidy and how a zygote with one extra or less chromosome usually would not survive to full term. I suppose this happens because aneuploidy leads to some kind of protein imbalance. So, in case of monosomy we would have a shortage of proteins, and in case of trisomy we would have an excess of proteins.

On the other hand, a conventional argument for having two copies of each gene is that, if one gene gets corrupted because of some kind of mutation, we still have another copy that produces the correct protein. So, in case of a recessive genetic disorder, the zygote can survive even with half the amount of protein.

My question is then, why can't monosomic aneuploidy be looked at the same way? Moreover, I read that monosomic aneuploidy is almost always fatal, while trisomic aneuploidy is sometimes tolarated.

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What causes zygotic termination in aneuploidy often isn't an issue with proteins or other gene products i.e. being "too sick" to live. It's that the zygote is basically "self error checking" and if it encounters a serious genetic error (e.g. aneuploidy) it self-terminates, as it would likely not survive anyway. Sometimes this self-termination fails, and the zygote develops. Depending on the specific aneuploidy, it may not make it to term, or perhaps it will, and you will have a child born, albeit the child will usually have a decreased quality of life/life-span.

A similar mechanism works in your somatic (non-reproductive) cells everyday when they replicate and divide - if they detect and error in their DNA that cannot be repaired, the cells SHOULD kill themselves. Sometimes this fails, and can give rise to cancer.

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    $\begingroup$ Thanks for your reply, can you add something more about the "self error checking" mechanism, maybe a link?. $\endgroup$ – Hemant Kumar Nov 6 '16 at 14:50
  • $\begingroup$ Just one doubt, but people with aneuploidy have problems because their aneuploidy translates into some sort of protein imbalance, and that brings me back to my question $\endgroup$ – Hemant Kumar Nov 6 '16 at 14:52
  • $\begingroup$ This sounds self-contradictory: you're saying that extra chromosomes are not harmful in themselves, but the cell detects them and self-terminates because extra chromosomes would be ... harmful? I'm no expert on this topic, but I would guess the main underlying problem is that aneuploidy causes problems in cell cycle/mitosis, which is lethal. Also, it probably would result in even more scrambled cells in the next generation (after meiosis), and so would endanger the whole genetic material. $\endgroup$ – Roland Nov 6 '16 at 15:25
  • $\begingroup$ @Roland Forgive me, my response was written past midnight. I meant to say that rather than progressing to the stage of dying from protein imbalance, the cell terminates before this point, usually. $\endgroup$ – GrumpyMammoth Nov 7 '16 at 5:48

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